Fans at maximum speed + battery issue after fixing "not turning on" problem

Hi,

A week ago, my MacBook Pro (Retina, 15-inch, Early 2013) suddenly stopped working: it would not turn on when pressing the power button. After trying solutions as SMC and PRAM reset, it was still impossible to turn it on. So I tried to open my MacBook, disconnected my battery, then I pressed a long time on the power button and it solved the problem: it was working again!
…But new problems have appeared:
1) Now the fans of the laptop is at maximum speed after booting even though no application is launched (no high %CPU usage in app monitor and the temperature of the MacBook components seems to be normal as well). Again, I tried SMC and PRAM reset but no results. I noticed that after an update of the system, the fans were stabilized again until I restarted the Mac: the fans were going crazy again.
I feel like I activated a mode in which the fans run at full speed like in this post
https://support.apple.com/en-sg/HT204463
because I may have done something similar (pressing the power button when connecting back the battery of my laptop).
2) The battery is now discharging even when the MacBook is off.

Has anybody had the same issues before and/or has any idea on how to solve it?

Thank you

Welcome to our forums.

The link you provided is information that pertains to an iMac not your MacBook Pro. An iMac does not have a battery and the SMC reset for it is different from that of your MacBook Pro. Also, the battery and associated circuitry in your MacBook Pro is non user serviceable and should only be accessed by Apple or an authorized Apple repair service center.

If there is an Apple Store nearby, we suggest taking your MacBook Pro to them and let them run diagnostics. The diagnostics are free at an Apple Store but repairs will have to be paid for if your MacBook Pro is out of warranty.